Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...416..733h&link_type=abstract
Astrophysical Journal v.416, p.733
Astronomy and Astrophysics
Astronomy
20
Convection, Hydrodynamics, Stars: Interiors
Scientific paper
Numerical simulation of compressible hydrodynamics in the context of astrophysical convection is necessarily restricted to a limited volume of an entire star: however, the physical parameters inside that volume are determined by what goes on throughout the star as a whole. The aim of boundary conditions (BCs) is to model the contact (both mechanical and thermal) between the computational domain and the rest of the star. Different investigators have used various combinations of BCs. Here we explore how the choice of BC affects certain aspects of the solutions of three-dimensional compressible convection. As regards mechanical BCs, we examine both closed and open domains. In the case of open domains, some of the BC sets we examine are based on recent suggestions by Poinsot & Lele for applying the method of characteristics to dissipative flows. Our results indicate that the hydrodynamic solutions can be qualitatively different depending on the choice of BC.
As a specific example of the differences, we examine how the BCs affect the relative contributions of acoustic and incompressible components to the overall flow. In recently reported simulations of homogeneous decaying turbulence, Ghosh & Mattheaus have found that the relative contribution of acoustic components depends sensitively on the choice of conditions in the initial state. Analogously, the ratio Rai of acoustic to incompressible power in convective simulations in statistically steady state might be expected to depend on the choice of certain conditions also: in convection, the controlling factors would be expected to be the boundary conditions. To investigate that possibility, it is necessary to treat the two modes of a stratified compressible medium (the gravity-driven and acoustic modes) on an equal footing: both modes are subject to instability (if radiative effects are at work) although the criteria for onset of instability are different for the two modes. (In the surface layers of the Sun, convective and acoustic modes seem to have comparable amplitudes, as if both modes may be experiencing instability.) We report results of an exploration of a small subset of BCs which indicate that the value of Rai does exhibit significant variation depending on BCs. In the astrophysical context, this conclusion has implications for the modeling of chromospheres in cool stars.
Hossain Murshed
Mullan Dermott J.
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